1. Field of the Invention
The present invention relates to a surface-emitting semiconductor laser, and more particularly, to a vertical-cavity surface-emitting laser diode (hereinafter referred to as VCSEL) having a mechanism for protecting the VCSEL from electrostatic discharge damage.
2. Description of the Related Art
The VCSEL emits light from the front surface of a semiconductor substrate, and several features such as low driving current, testing on the wafer level and easy mounting, as compared to the edge-emitting semiconductor laser. The VCSEL is utilized as various light sources such as a light source in optical information processing and optical communications and a light source for data storage.
A high voltage such as static electricity may be applied to the VCSEL at the time of mounting the VCSEL on a circuit board, as in the case of other types of semiconductor devices. When electrostatic discharge (hereinafter simply referred to as ESD) occurs within the device, large spike current flows instantaneously and may destroy or deteriorate the device. ESD may be a cause that malfunctions the device. Some reports directed to coping with ESD have been known.
Japanese Patent Application Publication No. 2000-216442 (Document 1) discloses a semiconductor light-emitting device having a semiconductor light-emitting element such as an LED, a first lead connected to the cathode electrode of the element, a second lead connected to the anode electrode of the element, and an electrostatic discharge damage preventing element electrically connected between the first and second leads such as a constant-voltage diode, these components being integrally sealed with resin.
U.S. Pat. No. 6,185,240 (Document 2) discloses an arrangement in which a diode against ESD and VCSEL are integrated on a single chip. A trench is formed in the substrate of the chip in order to define a diode region. The diode is preferably a p-i-n diode and has a breakdown voltage of 10-20 V.
The semiconductor light-emitting device disclosed in Document 1 is designed to separately have the semiconductor light-emitting element and the ESD preventing element. Thus, when the light-emitting element is solely handled, sufficient ESD protection may not be obtained. In addition, the device needs a fabrication step of mounting the element for preventing the electrostatic discharge damage, which increases the cost.
The VCSEL disclosed in Document 2 desired ESD protection because the VCSEL and the protection diode are integrated on the chip. However, part of the VCSEL is connected in parallel with the remaining VCSEL in the reverse direction in order to define the ESD protection diode, which needs multiple trenches in the substrate. This brings about a complicated structure.
Yet another ESD protection uses a dummy element connected in parallel to the VCSEL on the identical chip in order to improve the ESD breakdown voltage, as proposed by the same assignee as that of the present application. However, the proposed ESD protection allows drive current to flow through the dummy element and consumes a large amount of power. In addition, the measurement has a relative large electrostatic capacitance, which degrades responsiveness.